1. Field of the Invention
The present invention pertains to the regulation of power supplies with multiple outputs. In particular, this invention relates to a leading-edge modulation technique used for secondary-side post-regulation (SSPR) of power supplies with multiple outputs.
2. Description of Related Art
In response to the increasing complexity of many electronic devices, there has been an increasing demand for power supplies that provide two or more isolated and tightly regulated voltages. Most microprocessors, for example, require a precisely controlled supply voltage of 3.3V or lower as well as the standard supply of 5V. A recent trend towards smaller-sized electronic products has, however, introduced several design issues regarding the power density and efficiency of such power supplies. In order to address these issues, design engineers have a limited number of regulation techniques at their disposal. Some of the most popular techniques include the use of linear regulators, coupled inductors, post DC/DC (direct current/direct current) converters, magnetic amplifiers, and secondary-side post-regulators (SSPRs). Although each of these techniques has its own advantages and disadvantages, the high efficiency, low cost, and simple implementation of the SSPR technique has rapidly made it the most popular choice for high-frequency, high-power density DC/DC converters.
An SSPR uses a semiconductor device as a switch. This device is connected in series with the power converter""s secondary winding and performs either a delayed turn-on function (leading-edge modulation) or a delayed turn-off function (trailing-edge modulation). The transfer functions for both modes are the same except for a negative sign in front of the transfer function for the leading-edge modulation, because the greater the required duty cycle, the earlier the power switch must turn on. Although similar, these modes each possess unique functional characteristics. In particular, while leading-edge modulation is compatible with any pulse width modulator topology and any control method, trailing-edge modulation creates current waveforms on the primary side with a negative step. This negative step characteristic makes trailing-edge modulation incompatible with peak-current-mode control, which is by far the most popular mode of operation.
Although leading-edge modulation is well known in the art, its implementation is often complicated and usually consists of many components. Accordingly, this invention satisfies the need for a simplified leading-edge modulator for use in secondary-side post-regulation.
The present invention is directed to a simplified leading-edge modulator for use in secondary-side post-regulation of multiple output voltage power supplies.
In an embodiment of the invention, a multiple output voltage power supply comprises a transformer having a primary winding and a plurality of secondary windings. The primary winding is adapted to have an input voltage coupled thereto. A primary side switch is coupled to the primary winding and is adapted to control power applied to the transformer in a power cycle responsive to a peak current mode control signal. A plurality of output circuits are coupled to respective ones of the plurality of secondary windings and provide respective output voltages. The plurality of output circuits each further comprise a forward rectifier adapted to rectify power during a positive portion of the power cycle and a free-wheeling rectifier adapted to rectify power during a negative portion of the power cycle. One of the plurality of output circuits further comprises a bypass switch adapted to interrupt operation of the respective forward rectifier. A secondary side post regulation circuit controls operation of the bypass switch. The post regulation circuit further comprises a signal generator adapted to receive a synchronization signal corresponding to the power cycle. The signal generator generates a sawtooth waveform that rises during a positive portion of the synchronization signal and falls during a zero portion of the synchronization signal. A Schmidt trigger coupled to the signal generator has an enabled state when a leading edge of the sawtooth waveform reaches a threshold level and a disabled state when the synchronization signal falls to zero. A driver coupled to the Schmidt trigger generates a drive signal used to control the bypass switch in accordance with the enabled and disabled states of the Schmidt trigger.